Characters that wish to learn a new O.C.C. must first advance at least one level in their current O.C.C./R.C.C. At second level (or wherever desired), they may opt for training in the new O.C.C. as SOON as they reach that new level (it cannot be done after they have advanced in their new experience level).
When the character begins their new training, they are zero-level in the new O.C.C. The character must earn experience points equal to the new O.C.C.'s second level (if Men of arms or adventurers) or third level (if magical or psychic). When that amount of experience points is acquired, the character has passed their apprenticeship and is now at first level in his new O.C.C.
Once characters change over to their new O.C.C., they retain their old skills (but frozen at the levels they achieved before the change) but all new PPE, S.D.C., and other increases will be based upon their new O.C.C. ALL new experience points are awarded to the new O.C.C.